1. Field of the Invention
This invention relates to a method of positional detection using a pattern matching process. This invention also relates to an apparatus for positional detection using a pattern matching process.
2. Description of the Prior Art
It is well-known to use pattern recognition technologies in inspecting a wiring pattern and a parts-arrangement pattern on a printed circuit board, and positions of position-reference marks and positions of parts on a printed circuit board.
U.S. Pat. No. 4,776,023 discloses a pattern inspection method in which two kinds of images corresponding to a reference pattern and a pattern to be inspected are converted into binary images. Local images which are cut out from the binary images are compared with each other to detect differences between the cut-out images and to recognize these differences as a defect. Excess sensitivity to the different portions is moderated to the extent of allowing non-serious actual defects. By setting don't care areas, each of which consists of one pixel row neighboring on a binary boundary line in the image, and by comparing the remaining portions of the images other than the don't care areas by logical processing, it is possible to detect various defects without regarding the quantization error as a defect.
U.S. Pat. No. 4,641,355 discloses a pattern recognition apparatus which includes a memory for storing information relating to a known set of characters, means for defining a matrix of selected mesh pattern, means for comparing an unknown character with the known set of characters based on the mesh pattern, and means for storing information relating to the unknown character if it exceeds a selected minimum deviation from all known characters. Specifically, a bit matrix of the unknown pattern is obtained, and a determination is made as to minimum deviations between the cluster of characters of the unknown pattern and a group of registered cluster of characters on the basis of a stored given table. A selection is given of only an unknown pattern cluster of characters having minimum deviations exceeding a preset value.
In most of inspecting apparatus using pattern recognition, an object to be inspected is illuminated by a suitable lighting device, and an image of the object is picked up by a television camera. Due to various adverse factors such as undesirable reflection of light at a surface of the object, underexposure, or blooming, some area of the picked-up image of the object tends to be inaccurate in indicating the actual appearance of the corresponding area of the object. Such an area is referred to as an invalid area (an unreliable area). Generally, the invalid area lowers the accuracy of pattern recognition regarding the picked-up image.